The present invention is directed to the field of thermal cleaning with nitrogen trifluoride and an oxygen source etchant mixture. More specifically, the present invention is directed to cleaning various silicon-containing semiconductor substances from wafers and chemical vapor deposition equipment, including reaction vessels and hardware, using a dynamic flow of nitrogen trifluoride and an oxygen source at elevated temperatures with a unique process.
The semiconductor industry has experienced a long-felt need to solve the problem of cleaning chemical vapor deposition furnaces and quartz tubes in the furnaces as well as quartz hardware of various undesired substances such as silicon nitride, polycrystalline silicon, titanium silicide, tungsten silicide and various other silicides, as well as silicon dioxide, which are typically present as undesired films on furnaces and quartz hardware during their deposition on silicon wafers and chips being processed as electronic materials and integrated circuits.
The normal procedure for cleaning is to remove the parts, such as those made of quartz, metals or other materials from a furnace, such as quartz furnace tubes, and perform a wet chemical cleaning. O-ring seals would typically have to be replaced in such an operation, as well as cleaning of lines, doors and other vacuum components. Another alternative is to use another piece of equipment and perform a plasma cleaning. The wet chemical cleaning is very costly and time consuming. When the equipment is shut down to pull the quartz parts, it can require up to 18 hours of time. Also, the system necessitates reverification for its operating integrity before it can be used again. Direct cleaning application requires special chemicals, equipment and sinks to perform this cleaning. Another disadvantage is that the quartzware is attacked at accelerated rates which affects its reproducibility and reliability of operation. This is especially true when cleaning quartz racks or wafer holders. These components have special slots cut in the quarts to hold the parts being coated, and when the wet chemical clean attacks the quartz, it affects the dimensions of these slots. When the slots are affected, the parts being held are not coated uniformly, which requires the quartz be replaced at accelerated scheduling.
The plasma cleaning method requires the use of another piece of equipment especially designed to perform a cleaning of quartz tubes in place within the chemical vapor deposition furnace equipment. First, the plasma cleaning method does not clean the other quartz parts which are used in the chemical vapor deposition furnace system. This means these parts will need to be chemically wet cleaned, separate and apart from plasma cleaning methods. Also, the plasma cleaning equipment takes up space in the manufacturing area, and may prevent other tubes from being used in the system while it is being utilized. If the quartz parts are replaced with another material like silicon carbide, it will make the plasma system application unreliable.
Methods of using nitrogen trifluoride have been attempted but never have been brought to the marketplace. One such method was performed in a static mode that allowed by-products to condense on the cooler walls of the system. This caused a dangerous condition when these by-products were not evacuated before returning the system to the atmosphere. This called for extended purging times and reduced the benefits of this method. It also allowed for possible dangerous gas mixtures to develop in the system's vacuum components that may mix with gases which are normally utilized to deposit films on the wafers. One nitrogen trifluoride cleaning method is discussed in UK Patent Appln. GB2183204 where nitrogen trifluoride is used in a static mode and suggestion for use in a continuous flowing mode is also set forth. This patent application does not address the means or methods for removing cleaning by-products, the treatment of by-products once removed or the use of nitrogen trifluoride with any other gases.
U.S. Pat. No. 5,421,957 discloses a nitrogen trifluoride cleaning process for semiconductor process reactors and apparatus by controlling the moisture content during the clean operation to be less than 10 parts per million by volume. Inert carrier gases for the nitrogen trifluoride are disclosed such as nitrogen, helium, argon or the like. The concentration of the inert was from 95.5% to 80% of the etchant mixture.
U.S. Reissue Pat. No. 30,505 discloses a process for plasma etching of a solid material with a binary mixture comprising essentially oxygen and a halocarbon wherein at least one carbon atom in said molecule is linked to a predominance of fluorine atoms. At the temperatures recited (25.degree.-300.degree. C.) there is no reaction between the binary gas mixture and the solid material to be etched. Temperatures in excess of 1000.degree. C. are necessary to thermally dissociate halocarbons, making this gas impractical for thermal cleaning of semiconductor process equipment.
U.S. Pat. No. 4,374,698 describes an etch process for differential etching of silicon nitride from silicon dioxide using the combination of carbon tetrafluoride and a halofluorocarbon. The gas etchant may include oxygen or nitrous oxide. A plasma is necessary to dissociate the halocarbon into species that will react with the solid material. The role of the oxygen source in this patent is to volatilize the carbon products into CO and CO.sub.2. Without oxygen, this process would coat the process equipment with teflonlike material, defeating the usefulness of the process for cleaning.
U.S. Pat. No. 4,522,681 discloses a method for etching holes in silicon dioxide wherein a dry plasma etch gas of argon, nitrogen trifluoride and oxygen may be used. Polymeric photoresist materials such as polymethyl methacrylate, ethyl methacrylate, methyl isopropyl ketone as well as copolymers thereof with methacrylic acid may be used. This class of photoresist materials was required, versus standard novel AC photoresists, to successfully practice the invention. Plasma is necessary to dissociate the fluorine compound into specie that will etch the substrate. The role of the oxygen is to etch the photoresist and not the substrate.
U.S. Pat. No. 4,568,410 discloses a dry plasma etch process for etching silicon nitride using nitrogen trifluoride and oxygen. Good results for etching silicon nitride were found with relative percentages of the nitrogen trifluoride to oxygen recited at column 5, line 65 to be 10-20 SCCM of NF.sub.3 in comparison to 20-35 SCCM for oxygen. The gases are also disclosed as capable of etching common resists.
U.S. Pat. No. 4,787,957 is directed to a method for plasma desmear and etchbac of epoxy and polyimide materials from a multilayered or double sided printed circuit board using a plasma gas composition in the range of 20-45% NF.sub.3, the remainder being O.sub.2.
The prior art has failed to address a commercially successful process for cleaning semiconductor materials or equipment using a gaseous source to produce volatile cleaning by-products which are readily removed from the materials or equipment after cleaning is accomplished. In addition, the prior art has not addressed a viable method for removal of cleaning by-products and collection of such by-products (which typically may be reactive and toxic) for safe, easy disposal when servicing chemical vapor deposition equipment and materials treated in such equipment. The prior art has not addressed a method for using nitrogen trifluoride in combination with other gases so as to permit elevated furnace temperatures with controlled etch rates and with minimum down time for etch cleaning. The prior art has not utilized oxygen to enhance the thermal etch of semiconductor materials with nitrogen trifluoride. The present invention as set forth below overcomes these drawbacks of the prior art.